Teleportation breaks new ground

Jun 16, 2004

Physicists in Austria and the US have independently demonstrated quantum teleportation with atoms for the first time. Until now, teleportation had only ever been observed with photons. The results could represent a major step towards building a large-scale quantum computer.

In quantum teleportation, the sender, normally called Alice, instantaneously transfers information about the quantum state of a particle to a receiver called Bob. The uncertainty principle means that Alice cannot know the exact state of her particle. However, another feature of quantum mechanics called "entanglement" means that she can teleport the state to Bob.

Entanglement allows particles to have a much closer relationship than is possible in classical physics. If two particles are entangled, we can know the state of one particle by measuring the state of the other. For example, two particles can be entangled such that the spin of one particle is always "up" when the spin of the other is "down", and vice versa. An additional feature of quantum mechanics is that the particle can exist in a superposition of both these states at the same time.

David Wineland and colleagues from the National Institute of Standards and Technology (NIST) in Colorado began by creating a superposition of spin up and spin down states in a single trapped beryllium ion (Nature429 737). Using laser beams, they teleported these quantum states to a second ion with the help of a third, auxiliary ion (see figure). The NIST technique relied on being able to move the ions within the trap.

Meanwhile, Rainer Blatt and co-workers at the University of Innsbruck performed a similar experiment using trapped calcium ions (Nature429 734). However, rather than moving the ions, they "hide" them in a different internal state.

The success of a teleportation experiment is judged by its fidelity value -- a figure of merit that shows how faithfully the quantum state of the first system has been reproduced in the second system. Both the Innsbruck and NIST groups achieved fidelity values of around 75%. By comparison, approaches that do not use entanglement cannot achieve fidelity values above 66.6%.

"Teleporting the quantum state of an atom is important and exciting for scaling up quantum computers," Blatt told PhysicsWeb. "It can be applied to distributed quantum information processing, and together with interfacing techniques -- which are still under investigation -- for networking between different nodes in a quantum computer."